The present invention relates to a gas-rate sensor and more particularly to a temperature control means therefor.
A gas-rate sensor comprises a closed casing in which is sealed a gas, a pair of thermosensitive elements disposed within the casing, and a nozzle also arranged within the casing so as to confront the thermosensitive elements. In operation, when the electric currents flow through the respective thermosensitive elements to heat them, and simultaneously a gas is injected from the nozzle towards the heated thermosensitive elements within the casing, if the casing is in a state where no angular velocity is applied thereto the heated thermosensitive elements are both equally cooled by the injected gas, but when the casing is applied an angular velocity at this state the gas flow is deflected relative to the casing therein so that unequal cooling occurs between the two thermosensitive elements, resulting in generation of a minute electric potential difference between the two elements, and the measurement of the electric potential difference makes it possible to measure the angular velocity to which the casing is subjected. In this case, the gas is pressurized by an electrostriction oscillatory pump arranged within the casing at its one end, and after it is injected from the nozzle it is returned to the nozzle again so as to circulate within the casing.
Although a gas-rate sensor operates as described above, since it cannot ignore that the accuracy of the measured results are affected by the temperature of the atmosphere in which the casing is placed and thus the thermosensitive elements are also influenced by the atmospheric temperature, in order to suppress this effect to the minimum it has been already proposed to wind heating electric wires around the outer periphery of the casing so as to heat it from the outside, maintaining the temperature of the gas circulating within the case higher than that of the thermosensitive elements.
That is, this proposed gas-rate sensor comprises, as shown in FIG. 1 of the attached drawings, a hollow cylindrical casing 1 opened at both ends, the interior of the casing 1 sealed from the outside by closing the open ends with a pump holder 2 and a junction terminal board 3.sub.0, respectively, and the pump holder 2 constitutes an electrostriction oscillating pump 3 in association with a circular electrostriction transducer 3a made of electrostrictive ceramics which has its outer periphery integrally secured to the pump holder 2. And an electrode holder 4 is disposed within the casing 1 in parallel with and spaced from the electrostriction transducer 3a. Four electrodes 5a, 5b, 5c and 5d are mounted to the electrode holder 4 so as to be symmetric with respect to the central axis of the casing 1 although only the electrodes 5a and 5b are seen in the drawing, and the respective ends of the pairs of the electrodes 5a, 5b and 5c, 5d have thermosensitive elements 6a and 6b fused thereto so as to be in parallel with each other although only the thermosensitive element 6a can be seen in the drawing. The electrode holder 4 has also several gas induction holes 7 passed therethrough symmetrically with respect to the center axis of the casing 1.
Further, arranged within the casing 1 is a nozzle 8 near the junction terminal board 3.sub.0 so as to confront the electrode holder 4 and to be spaced therefrom. The nozzle 8 is provided centrally thereof with a nozzle orifice 9 with a number of auxiliary nozzles 10 being arranged in a circle at equi-angular intervals around orifice 9. A dust plate 11 is also provided within the casing 1 substantially midway between the nozzle 8 and the junction terminal board 3.sub.0.
The electrostriction pump 3 forms a pump chamber 12 between the surface of the electrostriction transducer 3a opposite to the pump holder 2 and the surface of the pump holder 2 itself, and upon oscillation of the electrostriction transducer 3a due to the supply of electricity to the electrostriction pump 3 from an outside electrical source, the gas is compressed within the pump chamber 12, the compressed gas being discharged from the pump chamber 12 through the discharge orifices 13 formed in the oscillator or electrostriction transducer 3a, and is guided through the flow passages 14 formed axially in the casing 1 from the gas induction holes 7 to a space formed between the nozzle 8 and the dust plate 11, and thence led through the nozzle orifice 9 and the auxiliary orifices 10 into the inside of a hollow cylindrical portion 15 formed centrally the casing 1, whereby the compressed gas is injected towards the electrodes 5a to 5d. After the gas equally cools the thermosensitive elements 6a and 6b respectively fused to the pairs of the electrodes 5a, 5b and 5c, 5d, it is exhausted towards the electrostriction transducer 3a from several discharge orifices 16 formed in the electrode holder 4. The exhausted gas is caused to be returned again to the nozzle 8 through the flow passages 14 by the action of the electrostriction pump 3 so that it is forced to be always circulated inside the casing 1.
The casing 1 is also provided with an IC- unit 20 outside the junction terminal board 3.sub.0 which is, on one hand, provided with terminals to receive electricity from the outside as well as to deliver the output signals of the thermosensitive elements 6a, 6b outside, and on the other hand is connected to the junction terminals 23 led from the junction terminal board 3.sub.0 outwards the casing 1 through lead pins 22 protruding towards the casing 1 from the outside.
The junction terminals 23 are adapted to be appropriately supplied with electricity necessary for the operation of the electrostriction oscillating pump 3 as well as the heating of the thermosensitive elements 6a, 6b from the IC-unit 20 through the lead pins 22 and a number of electric wires 24 which are connected to the lead pins 22 and passed through the casing 1 axially to the pump 3 and the thermosensitive elements 6a, 6b. The junction terminals 23 serve to take out the output signals issued from the thermosensitive elements 6a, 6a to the IC-unit 20 through the electrical wires 24 similarly disposed within the casing 1.
The casing 1 further has electrical heating wires 25 wound around its outer periphery over its hole length, the casing 1 together with the heating wire 25 being entirely covered by a hollow cylindrical heat insulating cover 26, and only the terminals of the IC-unit 20 protruding through the cover 26 to be exposed outside. In FIG. 1 the reference numeral 27 denotes a temperature sensor mounted outside the casing 1.
The operation of a conventional gas-rate sensor having a constitution as described above is as follows.
Upon supplying electricity from the IC-unit 20 to the electrostriction oscillating pump 3 mounted to the pump holder 2, the electrostriction transducer 3a is made to oscillate, and compresses the gas contained in the pump chamber 12. The compressed gas is made to flow through the discharge orifices 13 and the induction holes 7 into the flow passage 14 towards the dust plate 11, the gas reaching the nozzle 8 so that it is made to flow from the nozzle orifice 9 and the auxiliary nozzles 10 towards the electrode holder 4, finally passing through the discharge orifices 16. In this case, since the thermosensitive elements 6a and 6b fused to the ends of the pairs of the electrodes 5a, 5b and 5c, 5d, respectively, are exposed to the flow of the compressed gas the gas passes over the elements 6a and 6b to cool them equally. In this case, when an angular velocity is applied to the casing 1 the flow of the compressed gas within the casing 1 is deflected. Consequently the thermosensitive elements 6a and 6 b are cooled unequally by the gas, and the difference in temperature between them is output as a voltage. However, since the output voltage is minute it is amplified by the IC-unit 20 and taken out from the terminals 21 as an angular velocity signal.
Thus, since the conventional gas-rate sensor of this type is actuated by the difference in cooling between the thermosensitive elements 6a and 6b it is innevitably sensitive to temperature. Accordingly, it can be protected from the influence of the variations in outside temperature by controlling the temperature of the gas-rate sensor at a fixed temperature higher than the maximum temperature to which the sensor is expected to be subjected.
For this purpose, in the conventional gas-rate sensor, as described above, as a temperature control means the electrical heating wires 25 are wound around the outer periphery of the casing 1, and they are entirely surrounded by the heat insulating cover 26 to maintain warmth. In this case, the setting of the heating temperature is detected by the temperature sensor 27 separately mounted to the outer surface of the casing 1, and the temperature of the casing 1 is regulated to a fixed temperature on the basis of the detected value.
However, as described above, since the gas-rate sensor cannot remain uninfluenced by variations in the outside temperature there arise such problems that the environmental range of use of the gas-rate sensor is limited, etc. Therefore, in order to resolve the problems the temperature control means as described above has been principally adapted, but such a temperature control means takes a relatively long time before it reaches and stabilizes at a predetermined set temperature requiring considerable electricity.
That is, in a temperature control system of a conventional gas-rate sensor having such a constitution that electrical heating wires 5 are wound around the outer periphery of a casing 1 and it is kept warm by a heat insulating cover 26, in order to heat up the thermosensitive elements 6a and 6b, which are greatly influenced by temperature, to a predetermined temperature, first the casing 1 has its outer periphery heated by the electrical heating wires 25, and the inner peripheral surface is heated by the heat conducted from the outer periphery through the wall of the casing 1, the gas sealed within the casing 1 being heated then by the heated inner peripheral surface through heat transfer, and the thermosensitive elements 6a and 6b are heated by the heated gas to reach a predetermined temperature. Thus it will be appreciated that in a conventional heat control system for a gas-rate sensor it takes a long time before the sensor reaches a definite set temperature and has its temperature stabilized, and that until it reaches that temperature it remains in a state where its performance is unstable.